Conventional methods for manufacturing a hollow member made of light-metal such as an aluminum base alloy by hot extrusion are known, such as a method shown in FIG. 5. In this method, a light-metal material 1 molded into a solid billet is fed into a container 2 of an extruder under heating; a pressure is applied from the back (from the direction shown by an arrow A in the drawing) of the light-metal material 1 by a stem 3; and the light-metal material 1 is extruded from a die opening having a predetermined cross-sectional shape to the front (to the direction shown by an arrow B in the drawing) through a couple of hollow dies 4 provided in a die-holder 9 continuing to the container 2. Thus, a product of the hollow member 5 (a rectangular tube in this drawing example) is prepared.
In this method, a hollow die such as a bridge die, a porthole die, or a spider die is used as the couple of hollow dies 4. The porthole die as an example of the hollow die is shown in FIG. 6.
The couple of hollow dies 4 has an internal die 4a positioned at the billet side and an external die 4b positioned at the hollow member 5 side. Both dies 4a and 4b are fit to each other and used in an integrated manner.
The internal die 4a includes a plurality of entry ports 6 (the example in the drawing has four entry ports, but one of them is not shown) perforated at a peripheral portion thereof and includes an internal bearing 7a (mandrel) which protrudes toward the downstream direction (the external die 4b side) in the extrusion at the central portion. The external die 4b is provided with a recessed welding chamber 8 having an approximate cross shape corresponding to the respective entry ports 6 of the internal die 4a. The welding chamber 8 has an external bearing 7b of a hole passing through the external die 4b in the axial direction at the central part. The external bearing 7b is formed into a shape so that a gap with a specified shape (a thin-walled rectangular tube in this drawing example) can be formed when the internal bearing 7a of the internal die 4a is inserted into the external bearing 7b. Thus, the hollow member 5 having a cross-section corresponding to the gap shape can be prepared by extrusion.
The mechanism of extrusion using the couple of hollow dies 4 will be briefly described with reference to FIG. 6. The light-metal material 1 is pushed from the direction of the arrow A and is pressed into the four entry ports 6 of the external die 4b so as to be divided and to flow in the respective entry ports 6. Namely, the light-metal material 1 is divided into four parts 1a, 1b, 1c, and 1d. The divided parts 1a to 1d converge at the welding chamber 8 of the external die 4b after passing through the entry ports 6 and are welded to be unified again. The unified light-metal material 1 is extruded from a gap between the external face of the internal bearing 7a having a rectangular cross-section and the internal face of the external bearing 7b having a rectangular cross-section for receiving the internal bearing 7a with the gap in the direction of the arrow B. As a result, the hollow member (rectangular tube) 5 having a rectangular hollow cross-section corresponding to the gap shape is formed. Therefore, the resulting hollow member 5 has four edges of welding portions 5a. 
Namely, since the product of the hollow member 5 prepared by this method is extruded through the processes of dividing joining/welding which are not performed in a general method using a solid die, the hollow member 5 necessarily has the welding portions 5a corresponding to the number and position of the entry ports 6 of the couple of hollow dies 4. The metallurgical welding adhesion between the welding portions and bearing portions (non-welded portions) influences mechanical properties, such as tensile strength, proof stress, and elongation, of the hollow member, in particular, largely influences strength. Defects in the welding adhesion of the welding portions causes fracture and deformation during secondary fabrication or in use thereafter; thus, the quality may not be sufficiently guaranteed.
The extrusion using the bridge die has an advantage of that the bridge die has a life cycle longer than that of other hollow dies, but has a disadvantage of that the operation for ensuring the strength of the welding portions is difficult. For example, an aluminum base alloy can be used without causing problems in some products which are not required to have relatively high strength, such as JIS-3000 series and JIS-6000 series. However, in products which are required to have high strength, such as JIS-7000 series, it is very difficult to ensure enough strength of the welding portions because of the metallurgical properties of the aluminum base alloy. Furthermore, in the case of JIS-5000 series, it is believed in this field that the extrusion using the hollow die is impossible. Thus, even development has been abandoned.
In cooperation with such conventional conditions, no method suitable for previously evaluating the strength of the welding portions exists. Actually, the strength cannot be confirmed until a test such as a tube expansion test after the manufacturing is performed. Therefore, the lack of strength often occurs in products, and the yield ratio is low, which is a problem. When the lack of strength is found, the die shape or extruding conditions are altered according to experimental knowledge or trial and error. Such countermeasures lack in repeatability and versatility and cannot sufficiently and rapidly respond to new product shapes and prescribed properties manufactured for the first time. Furthermore, the fabricated dies are useless, which is extremely inefficient.
The present invention has been accomplished under such circumstances. It is an object of the present invention to realize and establish new extrusion technology for stably manufacturing a light-metal hollow member (product) having excellent mechanical properties by solving all the basic problems relating to strength of the welding portions in the extrusion using a hollow die such as a bridge die, and also efficiently manufacturing the product having a strength satisfying a required level at low cost.